Is that true? It seems odd that I would have to drive a ground rod for a 100A subpanel in my detached garage when if I were to install a 100A sub panel right next to my original panel (for more circuits) I would'nt be required to drive another ground rod.

Detached structures, yes. Same structure no.

The earth's potential can change relative to distance especially during lightning and this is part of the reason the 2nd ground rod is needed. I've seen large buildings blow electronics during a storm because of ground potential differences.